In order to separate a sheet from a sheet stack and feed the sheet from the stack, a number of devices have been utilized. Friction feed rolls or belts, vacuum sniffer tubes, and vacuum feed rolls are among the most common of these devices. Vacuum feed devices have an advantage in handling light weight or easily damaged sheets or documents since there is minimal scuffing or scrubbing of the document or sheet by the feeder. While vacuum feed rolls work well, in instances where sheet separation may be difficult and/or a more positive feed is desired, vacuum feed belts may be employed. However, vacuum based feed belt systems often experience relatively high misfeed rates due to premature engagement of the feed belt with the next succeeding sheet before the feeding of the previous sheet has been completed. For, as can be understood, as the sheet is fed forward the trailing edge of the sheet gradually uncovers the vacuum ports or apertures in the vacuum feed belt plenum. This causes the next sheet in the stack to be prematurely acquired by the vacuum belt and may result in a sheet jam or multifeed condition.
To obviate this tendency, various corrective devices have been employed. For example, vacuum hold down devices located adjacent the rear edge of the sheet stack which are actuated after the trailing edge of the sheet being fed has cleared the vacuum hold down ports, friction retard pads to prevent multiple fed sheets from reaching the take-away rolls downstream from the feeder, gate type feeders wherein a control gate is carefully adjusted to approximately one sheet thickness such that only one sheet will fit through the gate, and the like. However, devices of the aforementioned types have drawbacks. For example, vacuum hold down devices require precise timing controls in order to actuate the hold down device at the precise moment that the trailing edge of the sheet being fed leaves the vacuum hold down ports. Friction retard devices, due to contact with the sheets being fed, may degrade any printed matter on the sheets. Further, the frictional characteristics of the retard surface may change due to wear, humidity, etc. And, throat or gate control feeders often require constant adjustment of the throat dimension to accommodate changes in the type and thickness of sheets being fed.
The present invention seeks to obviate the aforementioned problems by providing an automatic vacuum controlled vacuum based belt feeder wherein once the sheet being fed is acquired by the take-away rolls, the vacuum is automatically shut off from the vacuum feed belt plenum to prevent multifeeds.